The present invention generally relates to a chemical mechanical polishing method and more particularly, relates to a method for improving the thickness uniformity on a semiconductor wafer during a chemical mechanical polishing process.
Apparatus for polishing thin, flat semi-conductor wafers is well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semi-conductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is xe2x80x9cplanarizedxe2x80x9d or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A perspective view of a typical CMP apparatus is shown in FIG. 1A. The CMP apparatus 10 consists of a controlled mini-environment 12 and a control panel section 14. In the controlled mini-environment 12, typically four spindles 16, 18, 20, and 22 are provided (the fourth spindle 22 is not shown in FIG. 1a) which are mounted on a cross-head 24. On the bottom of each spindle, for instance, under the spindle 16, a polishing head 26 is mounted and rotated by a motor (not shown). A substrate such as a wafer is mounted on the polishing head 26 with the surface to be polished mounted in a face-down position (not shown). During a polishing operation, the polishing head 26 is moved longitudinally along the spindle 16 in a linear motion across the surface of a polishing pad 28. As shown in FIG. 1A, the polishing pad 28 is mounted on a polishing disc 30 rotated by a motor (not shown) in a direction opposite to the rotational direction of the polishing head 26.
Also shown in FIG. 1A is a conditioner arm 32 which is equipped with a rotating conditioner disc 34. The conditioner arm 32 pivots on its base 36 for conditioning the polishing pad 38 for the in-situ conditioning of the pad during polishing. While three stations each equipped with a polishing pad 28, 38 and 40 are shown, the fourth station is a head clean load/unload (HCLU) station utilized for the loading and unloading of wafers into and out of the polishing head. After a wafer is mounted into a polishing head in the fourth head cleaning load/unload station, the cross head 24 rotates 90xc2x0 clockwise to move the wafer just loaded into a polishing position, i.e., over the polishing pad 28. Simultaneously, a polished wafer mounted on spindle 20 is moved into the head clean load/unload station for unloading.
A cross-sectional view of a polishing station 42 is shown in FIGS. 1B and 1C. As shown in FIG. 1B, a rotating polishing head 26 which holds a wafer 44 is pressed onto an oppositely rotating polishing pad 28 mounted on a polishing disc 30 by adhesive means. The polishing pad 28 is pressed against the wafer surface 46 at a predetermined pressure. During polishing, a slurry 48 is dispensed in droplets onto the surface of the polishing pad 28 to effectuate the chemical mechanical removal of materials from the wafer surface 46.
An enlarged cross-sectional representation of the polishing action which results form a combination of chemical and mechanical effects is shown in FIG. 1C. The CMP method can be used to provide a planner surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An outer layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide layer can be formed and removed repeatedly.
During a CMP process, a large volume of a slurry composition is dispensed. The slurry composition and the pressure applied between the wafer surface and the polishing pad determine the rate of polishing or material removal from the wafer surface. The chemistry of the slurry composition plays an important role in the polishing rate of the CMP process. For instance, when polishing oxide films, the rate of removal is twice as fast in a slurry that has a pH of 11 than with a slurry that has a pH of 7. The hardness of the polishing particles contained in the slurry composition should be about the same as the hardness of the film to be removed to avoid damaging the film. A slurry composition typically consists of an abrasive component, i.e, hard particles and components that chemically react with the surface of the substrate. For instance, a typical oxide polishing slurry composition consists of a colloidal suspension of oxide particles with an average size of 30 nm suspended in an alkali solution at a pH larger than 10. A polishing rate of about 120 nm/min can be achieved by using this slurry composition. Other abrasive components such as ceria suspensions may also be used for glass polishing where large amounts of silicon oxide must be removed. Ceria suspensions act as both the mechanical and the chemical agent in the slurry for achieving high polishing rates, i.e, larger than 500 nm/min. While ceria particles in the slurry composition remove silicon oxide at a higher rate than do silica, silica is still preferred because smoother surfaces can be produced. Other abrasive components, such as alumina (Al3O2)may also be used in the slurry composition.
The polishing pad 28 is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surface. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
In more recently developed semiconductor fabrication technologies, the requirement of wafer global and edge planarization for inter-layer-dielectric films is gaining more importance as the size of integrated circuits is continuously reduced. In such devices, a large number of layers are stacked with low-K dielectric films for forming ultra large scale integrated circuits. In the chemical mechanical polishing of low-K dielectric films, the characteristics of within wafer non-uniformity; the difficulty to control edge profile for either the low-K films or films such as fluorinated silicate glass; and the phenomena of wafer edge collapsing in higher interlayer dielectric films directly impact the dimension of metal lines and vias and as a result, the die yield. A large deviation in critical dimension due to poor within wafer uniformity leads to the separation of vias from metal lines.
One of such typical processing problem is shown in FIGS. 2A and 2B. Wafer 28 shown in FIG. 2A was covered with a silicon oxide layer prior to a CMP process. The thickness uniformity on the wafer surface is satisfactory as shown by the relatively few contour lines. To the contrary, after wafer 28 is chemical mechanically polished, shown in FIG. 2B, the contour lines greatly increases on an edge portion 50 which is indicative of a wafer edge collapsing defect. The wafer edge collapsing defect is normally caused by a cumulation of slurry solution along the edge portion of the polishing pad and as a result, the edge of the wafer has a higher removal rate than the center portion of the wafer.
Others have addressed the wafer edge collapsing problem by adjusting different processing parameters, i.e. by using low pressure and high speed, by using metal dummy filling, by using harder polishing pad, and by designing different polishing heads. However, none of these techniques have proven to be effective in eliminating the wafer edge collapsing problem caused by the uneven polishing of a wafer surface.
It is therefore an object of the present invention to provide a method for improving thickness uniformity on a semiconductor wafer during a chemical mechanical polishing process that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method for improving thickness uniformity on a semiconductor wafer during a chemical mechanical polishing process which does not require major modification of the process equipment.
It is a further object of the present invention to provide a method for improving thickness uniformity on a semiconductor wafer during a chemical mechanical polishing process by reducing slurry concentration along a peripheral region on the polishing pad.
It is another further object of the present invention to provide a method for improving thickness uniformity on a semiconductor wafer during a chemical mechanical polishing process by removing slurry from an edge portion of a polishing pad and reducing the slurry concentration.
It is still another object of the present invention to provide a method for improving thickness uniformity on a semiconductor wafer during a chemical mechanical polishing process by spraying deionized water onto a peripheral portion of a polishing pad and removing slurry solution.
It is yet another object of the present invention to provide a method for improving thickness uniformity on a semiconductor wafer during a chemical mechanical polishing process by mounting an additional spray nozzle near the edge of a polishing pad and spraying deionized water therefrom.
It is still another further object of the present invention to provide a method for improving thickness uniformity on a semiconductor wafer during a chemical mechanical polishing process by mechanically removing slurry solution from a peripheral region of a polishing pad and thus reducing the slurry concentration in the region.
In accordance with the present invention, a method for improving thickness uniformity on a semiconductor wafer during a chemical mechanical polishing process which can be carried out by the operating steps of rotating a polishing pad with a polishing surface facing upwardly; rotating a semiconductor wafer with an active surface facing downwardly; pressing the active surface of the semiconductor wafer against the top surface of the polishing pad while dispensing simultaneously a slurry solution onto the top surface of the polishing pad; and removing the slurry from a peripheral region of less than 10 mm wide on the top surface of the polishing pad simultaneously during the pressing step to reduce a concentration of the slurry in peripheral region.
The method for improving thickness uniformity on a semiconductor wafer during a CMP process may further include the step of removing the slurry by a hydraulic means, or the step of removing the slurry by a solvent spray, or the step of removing the slurry by a water spray. The method may further include the step of removing the slurry by spraying water onto an edge portion of the wafer that is about 5 mm wide. The method may further include the step of removing the slurry by a mechanical means, such as by a squeegee. The method may further include the step of removing the slurry by a squeegee that is pressed onto an edge portion of the polishing pad to a width of about 5 mm wide. The method may further include the step of mounting a spray nozzle adjacent to each polishing pad and aiming the nozzle at an edge portion of the polishing pad.
The present invention is further directed to a method for improving polishing uniformity on a semiconductor wafer during a chemical mechanical polishing process which can be carried out by the steps of providing a polishing pad that is mounted on a rotatable platform; mounting a solvent spray nozzle juxtaposed to the rotatable platform; rotating the polishing pad with a polishing surface facing upwardly; rotating a semiconductor wafer with an active surface facing downwardly; pressing the active surface of the semiconductor wafer against the top surface of the polishing pad while dispensing simultaneously a slurry onto the top surface of the polishing pad; and spraying a solvent onto an edge portion of the polishing pad that is less than 10 mm wide such that a concentration of the slurry solution in the edge portion is reduced.
The method for improving polishing uniformity on a semiconductor wafer during a CMP process may further include the step of spraying deionized water onto an edge portion of the polishing pad to remove the slurry. The method may further include the step of spraying a solvent onto an edge portion of the polishing pad that is about 5 mm wide, or the step of spraying deionized water at a pressure of between about 5 psi and about 20 psi onto the edge portion of the polishing pad. The method may further include the step of spraying deionized water for a time period of between about 60 sec and about 180 sec onto the edge portion of the polishing pad. The method may further include the step of providing a solvent spray nozzle including a spray arm, a spray head, a spray pump and a solvent supply.